The disclosure of this case incorporates by reference the entire contents of the aforesaid patent, and thus for purpose of simplicity of presentation does not duplicate herein the detailed circuits found therein. However, for purpose of convenient cross reference where applicable similar reference character designations are used herein.
In various industrial system installations for transmission of power from motors to various operating loads, it is essential to protect expensive industrial machinery, motors, etc. in the event something goes wrong, or load variations exceed specified limits. This may be done either by providing monitoring metering devices with visual scales or with corresponding automatically operable digital alarm switching devices used to control power or safety devices either on site or at remote stations.
In load control systems wherever human monitoring is required, there remains some chance for error. Thus, when load conditions are displayed upon a calibrated meter scale for example it is not unusual to misinterpret the reading either (1) by misunderstanding the scale factor, the scale terms, or the scale range, (2) by misreading the scale, or (3) by making errors in extrapolating scale readings into alternate specific load parameters. Many prior art meter scales for example are in arbitrary scale units and provided with calibration and correction charts for extrapolation and interpretation of readings. Particularly where alarms need be actuated for emergency conditions, there is not time for this. Even when timing is no problem this is a primary source of error because it involves human intervention and interpretation. Also, the load parameters of interest vary from horsepower, speed, torque, etc. in various installations.
Furthermore in metering and control circuits it is desirable to improve accuracy of readings and sensitivity. For accurate instrumentation, amplifiers and signal processing circuits or devices are always a source of potential error or nonlinearity. Thus, the simpler the data processing system is the more reliability and freedom from error. Also, the more accurate the readings can be detected in the first instance, the better. Most systems have a detection accuracy limitation and it is difficult indeed to carefully control very small changes in speed or loading upon a movable terminal load element such as a drill bit work tool, baling machine, etc. If this can be done, the system energy can be cut down significantly by employing motors of smaller rating and protecting them against overload. This becomes significantly important in today's energy saving environment.
Installation of prior art measuring devices to a large degree either require access to wiring or mechanical linkages and thus are difficult and expensive to retrofit and need to be special or custom tailored for use in different kinds of systems. Such would prohibit use in most cases since O.E.M. produced equipment generally cannot justify the cost of custom tailored load protection devices.